How many gallons of waterare in a area 30 inches long by 12 inches wide by 18 inches deep?

It's a 29 Gallon tank. You can figure out gallons from cube inches here: http://www.metric-conversions.org/volume/cubic-inches-to-us-liquid-gallons.htm or you can see a tank with same measurements here: http://www.myfishtank.com/ProductDetails.aspx?productID=7#